Precursor of positive electrode active material for nonaqueous electrolyte secondary batteries and production method thereof and positive electrode active material for nonaqueous electrolyte secondary batteries and production method thereof

ABSTRACT

Provided is a precursor of a positive electrode active material containing, in a reduced amount, impurities which do not contribute to a charge/discharge reaction but rather corrode a firing furnace and peripheral equipment and thus having excellent battery characteristics and safety, and production method thereof. 
     A method for producing a precursor of a positive electrode active material for nonaqueous electrolyte secondary batteries having a hollow structure or porous structure includes obtaining the precursor by washing nickel-manganese composite hydroxide particles having a particular composition ratio and a pore structure in which pores are present within the particles with an aqueous carbonate solution having a carbonate concentration of 0.1 mol/L or more.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/509,941 filed Jul. 12, 2019, which is a continuationapplication of U.S. patent application Ser. No. 15/129,159 filed Sep.26, 2016, which is a US national stage application of PCT InternationalApplication No. PCT/JP2015/057229 filed on Mar. 12, 2015, which is basedupon and claims the benefits of priority from Japanese PatentApplication No. 2014-069590 filed on Mar. 28, 2014, the entire contentsof which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a precursor of a positive electrodeactive material for nonaqueous electrolyte secondary batteries andproduction method thereof and a positive electrode active material fornonaqueous electrolyte secondary batteries and production methodthereof.

BACKGROUND ART

With the recent wide spread use of portable devices, such as mobilephones and notebook personal computers, there has been a strong demandto develop small, light secondary batteries having high energy density.Among secondary batteries satisfying such a demand are lithium-ionsecondary batteries using lithium, lithium alloy, metal oxide, or carbonas a negative electrode active material. Such lithium-ion secondarybatteries are being actively researched and developed.

A lithium-ion secondary battery that uses, as a positive electrodeactive material, lithium composite oxide, particularly, lithium-cobaltcomposite oxide (LiCoO₂), which is relatively easily synthesized,supplies a 4V-level high voltage. For this reason, such lithium-ionsecondary batteries are being commercialized as batteries having highenergy density. To obtain excellent initial capacity characteristics orcycle characteristics, there have been developed many batteries usinglithium-cobalt composite oxide. Various fruits have already beenproduced.

However, lithium-cobalt composite oxide (LiCoO₂) uses a cobalt compound,which is rare and expensive, as a raw material and therefore causes acost increase. For this reason, there is a demand for a cheaperalternative serving as a positive electrode active material. Reducingthe cost of a positive electrode active material and thus producing acheaper lithium-ion secondary battery is of great industrialsignificance, since it can contribute to the downsizing andweight-reduction of portable devices which are being currently widelyused.

Among new positive electrode active materials for lithium-ion secondarybatteries are lithium-manganese composite oxide (LiMn₂O₄) usingmanganese, which is cheaper than cobalt, and lithium-nickel compositeoxide (LiNiO₂) using nickel.

Lithium-manganese composite oxide is formed of cheap materials and hasexcellent thermal stability. Accordingly, it can be said to be apromising alternative material to lithium-cobalt composite oxide.However, lithium-manganese composite oxide has difficulty in meeting ademand to increase the capacity of lithium-ion secondary batteries,which has been raised year by year, since its theoretical capacity isonly about half that of LiCoO₂. As for lithium-nickel composite oxide,it has lower cycle characteristics than lithium-cobalt composite oxideand is more likely to lose battery performance when used or stored in ahigh-temperature environment.

On the other hand, lithium-nickel-manganese composite oxide has thermalstability and durability similar to those of lithium-cobalt compositeoxide and is a promising alternative to lithium-cobalt composite oxide.For example, Patent Literature 1 proposes, as a precursor of a positiveelectrode active material containing lithium-manganese-nickel compositeoxide, manganese-nickel composite hydroxide particles having amanganese-nickel ratio of substantially 1:1, an average particlediameter of 5 to 15 μm, a tap density of 0.6 to 1.4 g/mL, a bulk densityof 0.4 to 1.0 g/mL, a specific surface area of 20 to 55 m²/g, and asulfate group content of 0.25 to 0.45% by mass. Patent Literature 1 alsodiscloses, as a production method of the manganese-nickel compositehydroxide particles, causing a mixed aqueous solution of a manganesesalt and a nickel salt having a manganese-nickel atomic ratio ofsubstantially 1:1 to react with an alkali solution in an aqueoussolution having a pH of 9 to 13 in the presence of a complexing agent onan appropriate stirring condition while controlling the degree ofoxidation of manganese ions to a predetermined range and thencoprecipitating the resulting particles.

There has been also proposed a positive electrode active material whoseparticle structure is controlled so as to improve cycle characteristicsor output characteristics. For example, Patent Literature 2 disclosesnickel composite hydroxide consisting of approximately sphericalsecondary particles which are agglomerations of multiple primaryparticles and which have an average particle diameter of more than 7 μmand equal to or less than 15 μm. The nickel composite hydroxide has[(d90−d10)/average particle diameter] of 0.55 or less, and[(d90−d10)/average particle diameter] is an index indicating the extentof the particle size distribution. Patent Literature 2 also discloses apositive electrode active material for nonaqueous electrolyte secondarybatteries obtained using this nickel composite hydroxide. The positiveelectrode active material has an average particle diameter more than 8μm and equal to or less than 16 μm, and [(d90−d10)/average particlediameter], which is an index indicating the extent of the particle sizedistribution, is 0.60 or less. The positive electrode active materialincludes a shell and a hollow present inside the shell.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Application PublicationNo. 2004-210560

[Patent Literature 2] International Publication No. 2012/169274

SUMMARY OF INVENTION Technical Problem

While the particle structure of the manganese-nickel composite oxide isconsidered in Patent Literature 1, the reduction of the impurities isnot considered. Lithium-nickel-manganese composite oxide obtained usinga conventional production method contains impurities that do notcontribute to a charge/discharge reaction, such as sulfate groups orchlorine derived from the raw materials. Accordingly, in the productionof a battery, an excess negative electrode material in an amountequivalent to the irreversible capacity of the positive electrodematerial must be used in the battery. This results in reductions in thecapacities per weight and per volume of the entire battery. Also, excesslithium accumulated on the negative electrode as the irreversiblecapacity is problematic in terms of safety.

Further, chlorine remaining as an impurity may volatilize in the firingstep, corroding the firing furnace and peripheral equipment. Thus, theproduct may be contaminated with a foreign metal, causing ashort-circuit in the battery. Accordingly, the amount of chlorine mustbe reduced as much as possible.

As for the positive electrode active material having the hollowstructure disclosed in Patent Literature 2, the production processthereof involves neutralizing and crystallizing nickel compositehydroxide, and the neutralization and crystallization include a nucleiformation step of forming the nuclei of particles and a particle growthstep of growing the formed nuclei. The particle growth step involvescrystallization at a relatively low pH value and thereforedisadvantageously makes impurities, particularly, sulfate groups orchlorine more likely to remain. Further, the nuclei formation stepinvolves the crystallization of fine particles. Accordingly, even whenthe particles are grown in the subsequent particle growth step, highdensity will be difficult to achieve, making impurities more likely toremain within the particles.

In view of the problems with the related art, an object of the presentinvention is to provide a precursor of a positive electrode activematerial that allows for obtaining a nonaqueous electrolyte secondarybattery containing a reduced amount of impurities and having highcapacity, small irreversible capacity, and excellent coulomb efficiencyand reaction resistance, and production method thereof.

Solution to Problem

To solve the above problems, the inventors intensively considered areduction in the amount of impurities. As a result, the inventors foundthat nickel-cobalt-manganese composite hydroxide containing fewerimpurities could be obtained by washing nickel-manganese compositehydroxide having a particular composition and structure with an aqueouscarbonate solution and that use of this composite hydroxide as aprecursor allowed for the production of a positive electrode activematerial containing fewer impurities and allowing for obtainingexcellent battery characteristics, and then completed the presentinvention.

A method for producing a precursor of a positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention is a method for producing a precursor of a positive electrodeactive material for nonaqueous electrolyte secondary batteries having ahollow structure or porous structure. The method includes obtaining theprecursor by washing nickel-manganese composite hydroxide particlesrepresented by the general formula (1) and having a pore structure inwhich pores are present within the particles, with an aqueous carbonatesolution having a carbonate concentration of 0.1 mol/L or more,

Ni_(x)Co_(y)Mn_(z)M_(t)(OH)₂  General Formula (1)

where 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1; and M is atleast one element selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo,Hf, Ta, and W.

Preferably, a porosity measured by observing a cross-section of thenickel-manganese composite hydroxide particles is 15% or more.

Preferably, the aqueous carbonate solution is an aqueous solution of atleast one selected from potassium carbonate, sodium carbonate, potassiumbicarbonate, and sodium bicarbonate, and a pH of the aqueous carbonatesolution is 11.2 or more.

Preferably, the nickel-manganese composite hydroxide particles arewashed with the aqueous carbonate solution having a temperature of 15 to50° C.

Preferably, the nickel-manganese composite hydroxide particles areobtained by effecting neutralization and crystallization by charging amixed aqueous solution of a metal compound containing nickel andmanganese and optionally cobalt and the element M and an aqueoussolution containing an ammonium ion donor into a warmed reaction vesselwhile adding, to a reaction solution, a sufficient amount of an aqueousalkali metal hydroxide solution to maintain alkalinity as necessary, andin the neutralization crystallization, a nuclei formation step offorming nuclei and a particle growth step of growing the formed nucleiare separately performed by controlling a pH value of the reactionsolution.

Preferably, the pH value in the nuclei formation step is controlled soas to become 12.0 to 14.0 at a reference solution temperature of 25° C.,and the pH value in the particle growth step is controlled so as tobecome 10.5 to 12.5 at a reference solution temperature of 25° C. and tobe lower than the pH value in the nuclei formation step.

Preferably, the mixed aqueous solution contains a chloride of at leastone of nickel, manganese, and cobalt.

A precursor of a positive electrode active material for nonaqueouselectrolyte secondary batteries of the present invention is a precursorof a positive electrode active material for nonaqueous electrolytesecondary batteries having a hollow structure or porous structure. Theprecursor consists of nickel-manganese composite hydroxide particlesrepresented by the general formula (1) and having a pore structure inwhich pores are present within the particles. The precursor has asulfate group content of 0.4% by mass or less and a sodium content of0.035% by mass or less. Ni_(x)Co_(y)Mn_(z)M_(t)(OH)₂ . . . GeneralFormula (1) where 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1;and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V,Cr, Zr, Mo, Hf, Ta, and W.

Preferably, the precursor has a chlorine content of 0.1% by mass orless.

A method for producing a positive electrode active material fornonaqueous electrolyte secondary batteries of the present invention is amethod for producing a positive electrode active material for nonaqueouselectrolyte secondary batteries, the positive electrode active materialconsisting of lithium-nickel-manganese composite oxide that isrepresented by the general formula (2) and has a hollow structure orporous structure. The method includes a mixing step of mixing theprecursor of the positive electrode active material for nonaqueouselectrolyte secondary batteries with a lithium compound to obtain alithium mixture and a firing step of firing the lithium mixture in anoxidizing atmosphere at 800 to 1100° C. to obtainlithium-nickel-manganese composite oxide.

Li_(a)Ni_(x)Co_(y)Mn_(z)M_(t)O₂  General Formula (2)

where 0.95≤a≤1.20; 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1;and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V,Cr, Zr, Mo, Hf, Ta, and W.

Preferably, the lithium compound is at least one selected from a groupconsisting of a hydroxide, oxyhydroxide, oxide, carbonate, nitrate, andhalide of lithium.

A positive electrode active material for nonaqueous electrolytesecondary batteries of the present invention consists oflithium-nickel-manganese composite oxide particles represented by thegeneral formula (2) and having a hollow structure or porous structure.The positive electrode active material has a sulfate group content of0.4% by mass or less and a sodium content of 0.035% by mass or less.

Li_(a)Ni_(x)Co_(y)Mn_(z)M_(t)O₂  General Formula (2)

where 0.95≤a≤1.20; 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1;and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V,Cr, Zr, Mo, Hf, Ta, and W.

Preferably, the positive electrode active material for nonaqueouselectrolyte secondary batteries has a chlorine content of 0.05% by massor less.

A nonaqueous electrolyte secondary battery of the present invention usesthe positive electrode active material for nonaqueous electrolytesecondary batteries.

Advantageous Effects of the Invention

The present invention provides the precursor that allows for obtaining apositive electrode active material for nonaqueous electrolyte secondarybatteries containing a small amount of residual impurities and havinghigh capacity, small irreversible capacity, and excellent coulombefficiency and reaction resistance, and production method thereof. Theproduction method of this precursor is easy and productive and produces,in a small amount, impurities which corrode the firing furnace,peripheral equipment, and the like. Thus, the damage to the productionfacilities can be reduced. Further, the method for producing thepositive electrode active material of the present invention allows foreasily obtaining a positive electrode active material for nonaqueouselectrolyte secondary batteries having high capacity, small irreversiblecapacity, and excellent coulomb efficiency and reaction resistance andtherefore is extremely industrially valuable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a coin battery used to evaluate a battery.

DESCRIPTION OF EMBODIMENTS

Now, detailed description will be made on a precursor of a positiveelectrode active material for nonaqueous electrolyte secondary batteriesand production method thereof and a method for producing a positiveelectrode active material for nonaqueous electrolyte secondary batteriesusing this precursor according to the present embodiment. The embodimentbelow is only illustrative, and the present invention includes theembodiment, as well as forms obtained by making various changes ormodifications thereto on the basis of the knowledge of those skilled inthe art.

1. Method for Producing Precursor of Positive Electrode Active Materialfor Nonaqueous Electrolyte Secondary Batteries (1) Composition ofNickel-Manganese Composite Hydroxide Particles

A method for producing a precursor of a positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentembodiment (hereafter also simply referred to as the “precursor”)includes obtaining the precursor by washing nickel-manganese compositehydroxide particles represented by General Formula (1) below and havinga pore structure in which pores are present within the particles(hereafter also simply referred to as the “composite hydroxideparticles”), with an aqueous carbonate solution having a carbonateconcentration of a 0.1 mol/L or more.

Ni_(x)Co_(y)Mn_(z)M_(t)(OH)₂  General Formula (1)

where 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1; and M is atleast one element selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo,Hf, Ta, and W.

In General Formula (1), x represents the nickel content of the compositehydroxide particles and is 0.2≤x≤0.8, preferably 0.35≤x≤0.6, morepreferably 0.4≤x≤0.6. By using, as the precursor, the compositehydroxide particles having a nickel content in the above range, apositive electrode active material having excellent discharge capacitycan be obtained.

In General Formula (1), y represents the cobalt content of the compositehydroxide particles and is 0≤y<0.3, preferably 0.1≤y≤0.25, morepreferably 0.1≤y≤0.2. By using, as the precursor, the compositehydroxide particles having a cobalt content in the above range, apositive electrode active material having excellent discharge capacityand cycle characteristics can be obtained.

In General Formula (1), z represents the manganese content of thecomposite hydroxide particles and is 0.07<z≤0.8, preferably 0.1≤z≤0.6,more preferably 0.1≤z≤0.5. By using, as the precursor, the compositehydroxide particles having a manganese content in the above range, apositive electrode active material having excellent cyclecharacteristics and thermal stability can be obtained.

In General Formula (1), t represents the element M content of thecomposite hydroxide particles and is 0≤t≤0.1, preferably 00.07, morepreferably 00.05. By using, as the precursor, the composite hydroxideparticles having an M content in the above range, a positive electrodeactive material maintaining discharge capacity and having excellentcycle characteristics and thermal stability can be obtained.

Note that the composition ratio among nickel, cobalt, manganese, andelement M in the composite hydroxide particles (the precursor) is alsomaintained in lithium-nickel-manganese composite oxide particles (apositive electrode active material) to be discussed later.

(2) Internal Structure of Nickel-Manganese Composite Hydroxide Particles

Since the composite hydroxide particles used in the present embodimenthave a pore structure in which pores are present within the particles,it is possible to obtain lithium-nickel-manganese composite oxideparticles having a hollow structure or porous structure (hereafter alsosimply referred to as the “composite oxide particles”). A positiveelectrode active material consisting of the composite oxide particleshaving a hollow structure or porous structure contacts an electrolytesolution in a larger area and therefore exhibits excellent outputcharacteristics.

As used herein, the term “the composite hydroxide particles having apore structure in which pores are present within the particles” refersto particles having a structure in which many pores are present amongprimary particles forming secondary particles and also refers tocomposite hydroxide particles from which a positive electrode activematerial having a hollow structure or porous structure can be formedafter a firing step (to be discussed later). Examples of such compositehydroxide particles include composite hydroxide particles that consistof secondary particles each having a central portion consisting of fineprimary particles and a shell consisting of larger primary particlesthan the fine primary particles outside the central portion and thathave many pores among the fine primary particles, as disclosed in PatentLiterature 2. The composite hydroxide particles may include multiplecentral portions each consisting of fine primary particles. The entirecomposite hydroxide particles may have many pores among primaryparticles. The structures listed above may be combined. The “hollowstructure” of the positive electrode active material refers to astructure in which each particle includes a hollow consisting of acentral space and a shell outside the hollow, and “the porous structure”thereof refers to a structure in which pores are distributed throughoutthe particles.

The pore structure and the hollow structure or porous structure areidentified by observing cross-sections of the composite hydroxideparticles and composite oxide particles using a scanning electronmicroscope.

For the composite hydroxide particles having the pore structure and thecomposite oxide particles having the hollow structure or porousstructure, the porosities thereof measured by cross-sectionalobservation are 5% or more, preferably 15% or more, more preferably 15to 85%. Thus, it is possible to cause the obtained positive electrodeactive material to contact an electrolyte solution in a sufficientlylarge area without excessively reducing the bulk density of the positiveelectrode active material.

The porosity can be measured by observing any cross-section of thecomposite hydroxide particles or composite oxide particles using ascanning electron microscope and then analyzing the resulting image. Forexample, the porosity can be obtained by embedding multiple compositehydroxide particles or composite oxide particles in a resin or the like,subjecting these particles to cross-section polishing or the like toallow them to be cross-sectionally observed, then measuring pores withinthe secondary particles (hollows in a hollow structure or pores in aporous structure) as being black using image analysis software (WinRoof6.1.1 or the like), measuring dense portions in the secondary particleedges (shells in a hollow structure, or cross-sections of primaryparticles forming a pore structure or porous structure) as being white,and calculating the area of [black portion/(black portion+whileportion)] with respect to any 20 or more particles.

(3) Method for Producing Nickel-Manganese Composite Hydroxide Particles

Any method, including known conventional ones, may be used to producethe composite hydroxide particles used in the present embodiment as longas composite hydroxide satisfying General Formula (1) above and having apore structure in pores are present within the particles is obtained.

One example is a method of effecting neutralization and crystallizationby charging, into a warmed reaction vessel, a mixed aqueous solution ofa metal compound containing nickel and manganese and, optionally, cobaltand at least one element M selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr,Zr, Mo, Hf, Ta, and W and an aqueous solution containing an ammonium iondonor while properly charging a sufficient amount of an aqueous alkalimetal hydroxide solution to keep the reaction solution alkaline.

In the neutralization and crystallization, it is preferred to separatelyperform a nuclei formation step of forming nuclei and a particle growthstep of growing the nuclei by controlling the pH value of the reactionsolution. This results in the production of nickel-manganese compositehydroxide particles consisting of secondary particles which areagglomerations of primary particles and having a pore structure.

As used herein, the term “separately perform a nuclei formation step anda particle growth step” does not mean that a nuclei formation reactionand a particle growth reaction proceed in the same layer in the sameperiod as in the conventional continuous crystallization process, butrather means that the period in which a nuclei formation reaction (anuclei formation step) mainly occurs and the period in which a particlegrowth reaction (a particle growth step) mainly occurs are separatedclearly.

Further, in terms of the uniformity, stability, or the like of theparticle diameter, it is preferred to control the pH value in the nucleiformation step so as to become 12.0 to 14.0 at a reference solutiontemperature of 25° C. and to control the pH value in the particle growthstep so as to become 10.5 to 12.5 at a reference solution temperature of25° C. and so as to become lower than the pH value in the nucleiformation step. It is also preferred to control the pH value in theparticle growth step so as to become lower than the pH value in thenuclei formation step by 0.5 or more.

For the air atmospheres in the nuclei formation step and particle growthstep, it is preferred to use, for example, an oxidizing atmospherehaving an oxygen concentration of 1% by volume or more in the nucleiformation step and to change the oxidizing atmosphere to a mixedatmosphere of oxygen and inert gas having an oxygen concentration of 1%by volume or less in the middle of the particle growth step.

That is, by using the oxidizing atmosphere in the nuclei formation stepand the initial part of the particle growth step, a central portionconsisting of fine primary particles and including many small pores canbe formed. Also, by changing the oxidizing atmosphere to aweakly-oxidizing to non-oxidizing atmosphere in the subsequent particlegrowth step, it is possible to form a particle structure in which ashell consisting of larger tabular primary particles than the fineprimary particles is present outside the central portion. In the case ofthe composite hydroxide particles having such a particle structure, thefine primary particles forming the central portion are absorbed by theshell in the firing step for obtaining a positive electrode activematerial. Thus, composite oxide particles having a hollow structure areobtained.

Further, by adjusting the time at which the atmosphere is changed in theparticle growth step, it is possible to control the size of the hollowsof composite oxide particles having a hollow structure.

On the other hand, by using a weakly-oxidizing to non-oxidizingatmosphere, that is, an mixed atmosphere of oxygen and inert gas havinga oxygen concentration of 1% by volume or less in both the nucleiformation step and particle growth step, it is possible to obtaincomposite hydroxide particles having a pore structure in which primaryparticles formed in the nuclei formation step are grown and in whichmany pores are present among the primary particles. In the case of thecomposite hydroxide particles having such a particle structure, thepores are left and enlarged when the primary particles are sintered andgrown in the firing step for obtaining a positive electrode activematerial. Thus, lithium-nickel composite oxide particles (a positiveelectrode active material) having a porous structure are obtained.

Examples of the metal compound containing nickel, manganese, and cobaltinclude, but not limited to, sulfide, nitrate, and chloride. Amongthese, sulfide, or chloride is preferably used in terms of the ease ofdrainage, or the environmental load. Particularly in terms of theenvironmental load, a chloride of at least one of nickel, manganese, andcobalt is preferably used.

Examples of the metal compound containing at least one element Mselected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo, Hf, Ta, and Winclude, but not limited to, magnesium sulfate, calcium sulfate, sodiumaluminate, aluminum sulfate, titanium sulfate, peroxotitanic acidammonium, titanium potassium oxalate, vanadium sulfate, ammoniumvanadate, chromium sulfate, potassium chromate, zirconium sulfate,zirconium nitrate, ammonium molybdate, sodium tungstate, and ammoniumtungstate.

The alkaline metal hydroxide may be a known material, and examplesthereof include, but not limited to, sodium hydroxide and potassiumhydroxide.

Examples of the ammonium ion donor include, but not limited to, ammonia,ammonium sulfate, ammonium chloride, ammonium carbonate, and ammoniumfluoride.

The concentration of ammonium ions in the nuclei formation step andparticle growth step is preferably 3 to 25 g/L, more preferably 5 to 20g/L so that the concentration of metal ions is kept constant tostabilize the shape or particle diameter.

(4) Washing with Aqueous Carbonate Solution

A method for producing the precursor of the present embodiment involvesobtaining the precursor by washing nickel-manganese composite hydroxideparticles having a pore structure in which pores are present within theparticles, with an aqueous carbonate solution having a concentration of0.1 mol/L or more, preferably 0.15 to 2.00 mol/L, more preferably 0.20to 1.50 mol/L. By using an aqueous carbonate solution having aconcentration of 0.1 mol/L or more for washing, impurities,particularly, sulfate groups, chlorine, or the like in the compositehydroxide particles can be efficiently eliminated owing to the effect ofion exchange with carbonate ions in the aqueous carbonate solution. Ifthe composite hydroxide particles having a pore structure are washedwith water or an aqueous solution of an alkali metal such as sodiumhydroxide, which has been used conventionally, the impurities inside theparticles are difficult to eliminate. Washing with the aqueous carbonatesolution is effective.

The pH of the aqueous carbonate solution used for washing is preferably11.2 or more, more preferably 11.5 or more at a reference temperature of25° C. Use of an aqueous carbonate solution having a pH of 11.2 or moreallows sulfate groups or chlorine, which form acid, to be moreefficiently eliminated. Since an aqueous carbonate solution is used, theupper limit of the pH is on the order of 12.5 at a reference temperatureof 25° C.

The aqueous carbonate solution is preferably an aqueous solution of atleast one selected from potassium carbonate (K₂CO₃), sodium carbonate(Na₂CO₃), potassium bicarbonate, and sodium hydrogen carbonate. Sincelithium carbonate, calcium carbonate, and barium carbonate have lowwater-solubility, an aqueous solution in which a sufficient amount ofsuch carbonate is dissolved may not be obtained.

If sodium carbonate, for example, is used as carbonate, theconcentration of the aqueous solution is preferably 0.2 mol/L or more,more preferably 0.25 to 1.50 mol/L. Since sodium carbonate has highwater-solubility, use of an aqueous solution thereof having aconcentration of 0.2 mol/L or more allows impurities such as sulfategroups or chlorine to be more efficiently eliminated.

The temperature of the aqueous carbonate solution is preferably 15 to50° C. When the temperature of the aqueous solution falls within theabove range, ion exchange is activated, and the impurities areefficiently eliminated.

The amount of the aqueous carbonate solution used for washing may be anyamount as long as sufficient washing can be performed so that thesulfate group content of the composite hydroxide particles is 0.4% bymass or less and the sodium content thereof is 0.035% by mass or less.If an aqueous solution of sodium carbonate, for example, is used, theamount thereof is preferably 1000 mL or more, more preferably 2000 to5000 mL with respect to 1000 g of the nickel-manganese compositehydroxide. If the amount is 1000 mL or less, the carbonate ions may notsufficiently substitute for the impurity ions, and sufficient washingeffects may not be obtained.

Typically, washing with the aqueous carbonate solution is performed for0.5 to 2 hours. However, the washing time may be any length of time aslong as sufficient washing can be performed so that the sulfate groupcontent of the composite hydroxide particles becomes 0.4% by mass orless and the sodium content thereof becomes 0.035% by mass or less.

Examples of the washing method include 1) a typical washing method ofadding the nickel-manganese composite hydroxide particles to the aqueouscarbonate solution to make a slurry and then filtering the slurry and 2)a filter passage washing method of feeding a slurry including thecomposite hydroxide particles generated by effecting neutralization andcrystallization to a filter such as a filter press and then passing theaqueous carbonate solution through the filter. The filter passagewashing method is preferable, since it eliminates impuritiesefficiently, allows filtration and washing to be continuously performedin the same equipment, and is therefore productive.

After performing washing with the aqueous carbonate solution, washingwith pure water is performed optionally. Washing with pure water ispreferable, since it eliminates cation impurities such as sodium.

Washing with pure water may be performed using a typical method. Iffilter passage washing with the aqueous carbonate solution is performed,it is preferred to perform filter passage washing with the aqueouscarbonate solution and then continuously filter passage washing withpure water.

2. Precursor of Positive Electrode Active Material for NonaqueousElectrolyte Secondary Batteries

A precursor of a positive electrode active material for nonaqueouselectrolyte secondary batteries of the present embodiment consists ofnickel-manganese composite hydroxide particles represented by GeneralFormula (1) below and having a pore structure and has a sulfate groupcontent of 0.4% by mass or less and a sodium content of 0.035% by massor less.

Ni_(x)Co_(y)Mn_(z)M_(t)(OH)₂  General Formula(1)

where 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1; and M is atleast one element selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo,Hf, Ta, and W.

Sulfate groups contained in the precursor are not reduced in the firingstep for producing a positive electrode active material but ratherremain in the positive electrode active material. For this reason, thesulfate groups need to be sufficiently reduced in the precursor inadvance. By using the precursor having a sulfate group content of 0.4%by mass or less, preferably 0.35% by mass or less, more preferably 0.3%or less by mass, a positive electrode active material having a sulfategroup content of 0.4% by mass or less, preferably 0.35% by mass or lesscan be obtained.

Sodium contained in the precursor is also not reduced in the firing stepfor producing a positive electrode active material but rather may beincreased due to lithium salt, which is a mixed raw material. For thisreason, the sodium content of the precursor needs to be sufficientlyreduced in advance so that it becomes 0.035% by mass or less, preferably0.030% by mass or less, more preferably 0.028% by mass or less.

The chlorine content of the precursor is preferably 0.1% by mass orless, more preferably 0.05% by mass or less, even more preferably 0.02%by mass or less. While chlorine has less influence on a positiveelectrode active material than sulfate groups, it has an adverse effecton a firing furnace or the like during the production of a positiveelectrode active material. For this reason, chlorine is preferablysufficiently reduced in the precursor in advance.

The sulfate group content, sodium content, and chlorine content of theprecursor can be set to the above ranges by properly adjusting theconcentration, amount, temperature, or the like of the aqueous carbonatesolution with which the nickel-manganese composite hydroxide is washed.

3. Method for Producing Positive Electrode Active Material forNonaqueous Electrolyte Secondary Batteries

A method for producing a positive electrode active material fornonaqueous electrolyte secondary batteries of the present embodimentincludes 1) a mixing step of mixing the precursor and a lithium compoundto obtain a lithium mixture and 2) a firing step of firing the lithiummixture at 800 to 1100° C. in an oxidative atmosphere to obtainlithium-nickel-manganese composite oxide particles represented byGeneral Formula (2) below.

Li_(a)Ni_(x)Co_(y)Mn_(z)M_(t)O₂  General Formula (2)

where 0.95≤a≤1.20; 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1;and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V,Cr, Zr, Mo, Hf, Ta, and W.

The respective steps will be described below.

(1) Mixing Step

The mixing step is a step of mixing the precursor and a lithium compoundto obtain a lithium mixture.

Preferably, the mixing ratio between the precursor and lithium compoundis adjusted so that the molar ratio of lithium (Li) to metal elements(Me) other than lithium (Li/Me) is 0.95 to 1.20, preferably 1.00 to1.15. That is, the precursor and lithium compound are mixed in such amanner that the molar ratio (Li/Me) of the lithium mixture becomes thesame as the molar ratio (Li/Me) of a positive electrode active materialof the present embodiment. The reason is that the molar ratio (Li/Me)does not vary between before and after the firing step and therefore themolar ratio (Li/Me) of the lithium mixture obtained in the mixing stepbecomes the molar ratio (Li/Me) of the positive electrode activematerial. Note that the value of a in General Formula (2) is similar tothe molar ratio (Li/Me) of the positive electrode active material, sincethe composition ratio (x+y+z+t) of Me is 1.

If the positive electrode active material obtained has a molar ratio(Li/Me) of less than 0.95, the battery capacity in charge/dischargecycles may be reduced. Also, if the molar ratio is more than 1.20, thepositive electrode of a battery produced using the positive electrodeactive material would have larger internal resistance.

Examples of the lithium compound include, but not limited to, at leastone selected from a group consisting of a hydroxide, oxyhydroxide,oxide, carbonate, nitrate, and halide of lithium. A hydroxide and/orcarbonate of lithium are more preferably used.

The mixing step is performed using a dry mixer such as a V blender, amixing/granulation apparatus, or the like.

A roasting step may be additionally performed before the mixing step.

Specifically, while the precursor, which is composite hydroxide, may bemixed with the lithium compound, the remaining water in the precursormay be eliminated by additionally performing a roasting step before themixing step. The composite hydroxide may also be converted intonickel-manganese composite oxide by performing a roasting step.

By performing the roasting step, it is possible to more easily controlthe ratio between lithium and the metal elements other than lithium, aswell as to stabilize the composition of lithium-nickel-manganesecomposite oxide particles to be obtained and to suppress thenon-uniformity of the composition in the firing step. For example, theroasting step involves firing the precursor at 150 to 1000° C. in anoxidizing atmosphere. To convert the precursor into nickel-manganesecomposite oxide, the precursor is roasted preferably at 400 to 900° C.,more preferably at 400 to 800° C.

If the roasting temperature is less than 150° C., the remaining water inthe precursor may not be sufficiently eliminated. If the roastingtemperature is more than 1000° C., lithium-nickel-manganese compositeoxide particles having low crystallinity may be produced. This isbecause such roasting temperature may abruptly grow the primaryparticles forming the particles of the precursor and thus mayexcessively reduce the reaction area of the roasted precursor, resultingin a reduction in the reactivity with lithium. On the other hand, theroasting step at a temperature of 300° C. or less can serve also as adrying step in a crystallization step for obtaining composite hydroxideparticles.

(2) Firing Step

The firing step is a step of firing the obtained lithium mixture at 800to 1100° C., preferably at 850 to 1050° C., more preferably 900 to 1000°C. in an oxidizing atmosphere. That is, while thermal treatment at atemperature more than 800° C. results in the production oflithium-nickel-manganese composite oxide, thermal treatment at atemperature less than 800° C. results in the production of that whosecrystal is undeveloped and whose structure is unstable and is easilybroken due for example to phase transition caused by a charge/discharge.Also, thermal treatment at a temperature more than 1100° C. results inthe production of lithium-nickel-manganese composite oxide whichincludes abnormally grown particles and has a collapsed layeredstructure and a low specific surface area, making it difficult to insertand desorb lithium ions.

Note that when the temperature is raised to 800° C. or more in thefiring step, the nickel-manganese composite hydroxide is converted intocomposite oxide and that the reaction between the precursor and thelithium compound substantially ends at about 700° C. For this reason,firing at up to 700° C. and firing at 700° C. or more may be performedin different equipment or as different steps. By performing firing astwo different steps, it is possible to prevent gas components, such aswater vapor and carbon dioxide, generated by the reaction between thenickel-manganese composite hydroxide and lithium compound from beingsubjected to firing at 700° C. or more, which aims to increasecrystallinity. Thus, composite hydroxide particles having highercrystallinity can be obtained.

Further, after eliminating crystal water or the like from the lithiumcompound, the lithium mixture may be kept at a temperature in a rangeof, for example, 600 to 900° C. and lower than the firing temperaturefor one hour or more and then calcined before fired so that the lithiummixture uniformly reacts in a temperature range in which crystal growthproceeds

The firing step is performed using a firing furnace, such as an electricfurnace, kiln, tube furnace, or pusher furnace, adjusted to a gasatmosphere having an oxygen concentration of 20% by mass or more, suchas an oxygen atmosphere or a dehumidified, decarbonated dry airatmosphere.

By using the precursor of the present embodiment as a raw material andusing the above production method, there can be obtained an positiveelectrode active material for nonaqueous electrolyte secondary batteriesconsisting of lithium-nickel-manganese composite oxide particles havinga sulfate group content of 0.4% by mass or less and a sodium content of0.035% by mass or less, and, preferably, having a chlorine content of0.05% by mass or less.

4. Positive Electrode Active Material for Nonaqueous ElectrolyteSecondary Batteries

A positive electrode active material for nonaqueous electrolytesecondary batteries of the present embodiment consists oflithium-nickel-manganese composite oxide particles represented byGeneral Formula (2) below and having a hollow structure or porousstructure and has a sulfate group content of 0.4% by mass or less and asodium content of 0.035% by mass or less.

Li_(a)Ni_(x)Co_(y)Mn_(z)M_(t)O₂  General Formula (2)

where 0.95≤a≤1.20; 0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1;and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V,Cr, Zr, Mo, Hf, Ta, and W.

In General Formula (2), x, y, z, and t can take values similar to x, y,z, and t in General Formula (1) representing the composite hydroxideparticles described above, and a can take a value similar to Li/Me mixedin the mixing step.

The sulfate group content of the positive electrode active material is0.4% by mass or less, preferably 0.35% by mass or less, more preferably0.3% by mass or less. By using this positive electrode active material,it is possible to obtain a battery having reduced irreversible capacity,increased coulomb efficiency, and increased capacity. It is alsopossible to suppress excess lithium to be accumulated on the negativeelectrode and thus to improve safety.

The sodium content of the positive electrode active material is 0.035%by mass or less, preferably 0.030% by mass or less. As with sulfategroups, sodium also increases the irreversible capacity. By using thepositive electrode active material having a sodium content of 0.035% bymass or less, it is possible to obtain a battery having reducedirreversible capacity and increased coulomb efficiency. Also, sodiumremaining in the positive electrode active material increases theresistance value of the positive electrode active material. Accordingly,by using the positive electrode active material having a sodium contentin the above range, the output characteristics can be improved.

Chlorine also increases the irreversible capacity. For this reason, thechlorine content of the positive electrode active material is preferably0.05% by mass or less, more preferably 0.01% by mass or less, even morepreferably 0.005% by mass or less.

The lithium-nickel-manganese composite oxide particles forming thepositive electrode active material of the present embodiment have ahollow structure or porous structure. Thus, the positive electrodeactive material can contact an electrolyte solution in a larger area andtherefore can exhibit increased output characteristics. To furtherimprove the output characteristics, the porosity measured bycross-sectional observation is preferably 15% or more, more preferably15 to 85%. The particle structure or porosity can be identified orobtained in a manner similar to that for the composite hydroxideparticles described above.

5. Nonaqueous Electrolyte Secondary Battery

As with a typical lithium-ion secondary battery, a nonaqueouselectrolyte secondary battery of the present embodiment includeselements, such as a positive electrode, a negative electrode, and anonaqueous electrolyte solution. The respective elements will bedescribed in detail below. The embodiment below is only illustrative,and nonaqueous electrolyte secondary batteries of the present inventioninclude the embodiment, as well as forms obtained by making variouschanges or modifications thereto on the basis of the knowledge of thoseskilled in the art. The nonaqueous electrolyte secondary battery of thepresent embodiment may be used for any application.

(1) Positive Electrode

A positive electrode mixture material for forming a positive electrodeand materials included in the mixture material will be described. Theabove particulate positive electrode active material, a conductivematerial, and a binder are mixed. Optionally, activated carbon and asolvent for viscosity adjustment or other purposes are added and kneadedto prepare a positive electrode mixture material paste. The mixing ratioamong the components of the positive electrode mixture material may beadjusted in accordance with the required performance of a lithiumsecondary battery.

For example, as with the positive electrode of a typical lithiumsecondary battery, the positive electrode mixture material paste maycontain 60 to 95% by mass of the positive electrode active material, 1to 20% by mass of the conductive material, and 1 to 20% by mass of thebinder with respect to all the mass of the solid content of the positiveelectrode mixture material except for the solvent of 100% by mass.

By applying the positive electrode mixture material paste obtained, forexample, onto a collector formed of an aluminum foil, drying it todisperse the solvent, and optionally pressing it by roll press toincrease the electrode density, a sheet-shaped positive electrode can beobtained. The sheet-shaped positive electrode thus obtained can be usedin the target battery, for example, by cutting it into a size suitablefor the battery. The method for producing a positive electrode describedabove is only illustrative, and other methods may be used.

Examples of the conductive material used to produce the positiveelectrode include carbon black-based materials, such as graphite(natural graphite, artificial graphite, expanded graphite, etc.),acetylene black, and Ketjen black.

The binder has a function of binding active material particles together.Examples thereof include fluorine-containing resins, such aspolyvinylidene fluoride, polytetrafluoroethylene, ethylene propylenediene rubber, and fluororubber, and thermoplastic resins, such asstyrene butadiene, cellulose-based resin, polyacrylic acid,polypropylene, and polyethylene.

Optionally, a solvent for dispersing the positive electrode activematerial, conductive material, activated carbon, and the like anddissolving the binder may be added to the positive electrode mixturematerial. For example, an organic solvent, such asN-methyl-2-pyrrolidone, may be used as the solvent. Further, activatedcarbon may be added to the positive electrode mixture material toincrease the electric double layer capacity.

(2) Negative Electrode

A negative electrode is formed by mixing a binder with metal lithium,lithium alloy, or the like or a negative electrode active material whichcan occlude and desorb lithium ions, adding an appropriate solvent tothe mixture, applying the resulting pasty negative electrode mixturematerial onto a collector formed of a metal foil, such as copper, dryingthe mixture material, and optionally compressing it to increase theelectrode density.

Examples of the negative electrode active material include naturalgraphite, artificial graphite, a fired body of an organic compound suchas phenol resin, and the powder of a carbon material such as coke. As inthe positive electrode, the binder used in the negative electrode may bea fluorine-containing resin, such as polyvinylidene fluoride. Thesolvent for dispersing the negative active material and binder may be anorganic solvent, such as N-methyl-2-pyrrolidone.

(3) Separator

A separator is sandwiched between the positive electrode and negativeelectrode. The separator separates the positive electrode and negativeelectrode and holds an electrolyte. It may be a thin, porous film formedof polyethylene, polypropylene, or the like.

(4) Nonaqueous Electrolyte Solution

A nonaqueous electrolyte solution is prepared by dissolving a lithiumsalt serving as a supporting electrolyte in an organic solvent. Theorganic solvent may be, for example, one or combinations of two or moreselected from cyclic carbonates such as ethylene carbonate, propylenecarbonate, butylene carbonate, and trifluoropropylene carbonate, chaincarbonates such as diethyl carbonate, dimethyl carbonate, ethylmethylcarbonate, and dipropyl carbonate, ether components such astetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, sulfurcompounds such as ethyl methyl sulfone and butanesultone, phosphoruscompounds such as triethyl phosphate and trioctyl phosphate, and thelike.

Examples of the supporting electrolyte include LiPF₆, LiBF₄, LiClO₄,LiAsF₆, LiN(CF₃SO₂)₂, and composite salts thereof.

The nonaqueous electrolyte solution may contain a radical scavenger, asurfactant, a flame retardant, or the like.

(5) Shape and Configuration of Battery

The nonaqueous electrolyte secondary battery may take various shapes,including a cylindrical shape and a multilayer shape.

Whatever form it may take, the nonaqueous electrolyte secondary batterycan be completed by stacking the positive electrode and negativeelectrode with the separator therebetween to form an electrode body,impregnating this electrode body with the nonaqueous electrolytesolution, connecting a positive electrode collector and a positiveelectrode terminal leading to outside and connecting a negativeelectrode collector and a negative electrode terminal leading to outsideusing collection leads or the like, and hermetically sealing thesecomponents in a battery case.

EXAMPLES

Hereafter, the present invention will be described in more detail usingExamples and Comparative Examples. However, the present invention is notlimited to these Examples. The metals of nickel-manganese compositehydroxides/lithium-nickel-manganese composite oxides used in Examplesand Comparative Examples were analyzed and evaluated using the followingmethods.

(1) Composition analysis: the composition was measured by ICPspectrometry.(2) Sulfate group content: sulfur was quantitatively analyzed by ICPspectrometry and then the sulfate group content was obtained bymultiplying the amount of sulfur by a coefficient assuming that allsulfur was oxidized to become sulfate groups (SO₄ ²⁻).(3) Na and Cl contents: the Na and Cl contents were measured by atomicabsorption spectrometry.(4) Charge/discharge capacity and coulomb efficiency:

A coin-type battery was produced and left alone for about 24 hours;after an open circuit voltage (OCV) was stabilized, the battery wascharged to a cut-off voltage of 4.3 V with the current density withrespect to the positive electrode set to 0.5 mA/cm2; after left at restfor one hour, the battery was discharged to a cut-off voltage of 3.0 V;the then capacity was defined as the discharge capacity; and the ratioof the discharge capacity to the then charge capacity (dischargecapacity/charge capacity) was defined as the coulomb efficiency (%).

(5) Reaction resistance:

The reaction resistance was obtained by charging a coin-type battery ata charge potential of 4.1 V, creating an AC impedance Nyquist plot usinga frequency response analyzer and a potentio-galvanostat (1255Bavailable from Solartron), and calculating the value of the positiveelectrode resistance by performing fitting calculation using anequivalent circuit.

Example 1 Production of Precursor of Positive Electrode Active MaterialNuclei Formation Step

Water was charged into a reaction vessel (34 L) until the water fillshalf the volume thereof, and the temperature in the vessel was set to40° C. while stirring the water using an inclined paddle-type impellerat 500 rpm. At this time, an air atmosphere (oxygen concentration: 21%by volume) was used in the reaction vessel. Proper amounts of a 25% bymass sodium hydroxide aqueous solution and 25% by mass ammonia waterwere added to the water in the reaction vessel. The pH value of thereaction solution in the vessel was adjusted so as to become 13.0 at areference solution temperature of 25° C., and the concentration ofammonia in the reaction solution was adjusted so as to become 15 g/L.Thus, a pre-reaction aqueous solution was obtained.

Then, nickel sulfate, manganese sulfate, and cobalt chloride (the molarratio among the metal elements Ni:Co:Mn=50:20:30) were dissolved inwater, and the resulting 2.0 mol/L mixed aqueous solution was added tothe pre-reaction aqueous solution in the reaction vessel at a rate of 88mL/min to give a reaction aqueous solution. Simultaneously, 25% by massammonia water and a 25% by mass sodium hydroxide aqueous solution wereadded to the reaction aqueous solution at a constant speed.Crystallization (nuclei formation) was effected for 15 seconds whilecontrolling the pH value to 13.0 (the pH value for nuclei formation)with the ammonium concentration of the reaction aqueous solution (theaqueous solution for nuclei formation) kept at the above value.

Particle Growth Step

After forming nuclei, only the supply of the 25% by mass sodiumhydroxide aqueous solution was temporarily stopped until the pH value ofthe reaction aqueous solution became 11.6 at a reference solutiontemperature of 25° C.

After the pH value of the reaction aqueous solution reached 11.6, thesupply of the 25% by mass sodium hydroxide aqueous solution to thereaction aqueous solution (the aqueous solution for particle growth) wasresumed. With the ammonium concentration kept at the above value andwith the pH value controlled to 11.6 at a solution temperature of 25°C., crystallization was continued for 30 min to grow particles. Then,the supply of the solution was temporarily stopped, and a nitrogen gaswas circulated at 5 L/min until the oxygen concentration of the space inthe reaction vessel became 0.2% by volume or less. Then, the supply ofthe solution was resumed. Thus, crystallization was effected for a totalof 2 hours after the start of growth.

When the reaction vessel was filled with the solution, crystallizationand stirring were stopped, and the solution was left to stand. Thus, theprecipitation of the product was facilitated. Subsequently, a half ofthe supernatant was extracted from the reaction vessel and thencrystallization was resumed and effected for two hours (particle growth:a total of four hours) and then ended. The product was washed, filtered,and dried to give composite hydroxide particles.

During the crystallization, the pH was controlled by adjusting thesupply flow rate of the aqueous solution of sodium hydroxide using a pHcontroller. The variation width was within a range of ±0.2 of the setvalue.

The resulting composite hydroxide particles had a pore structure,consisted of spherical secondary particles having an average particlediameter of 9.3 μm which were agglomerations of primary particles havingdiameters of 1 μm or less, and had a porosity of 51%. For thecomposition thereof, the molar ratio among nickel, cobalt, and manganesewas 50:20:30.

Washing with Carbonate

The composite hydroxide particles were separated into solid and liquidcomponents using a filter press, then washed with 0.28 mol/L of anaqueous solution of sodium carbonate having a temperature of 25° C. anda pH of 11.5 (at a reference temperature of 25° C.) by passing theaqueous solution through the filter press at a rate of 3000 mL withrespect to 1000 g of the composite hydroxide particles, and furtherwashed with pure water by passing it through the filter press. Table 1shows the results, including the composition of the washednickel-cobalt-manganese composite hydroxide (precursor) and the amountof impurities.

Production of Positive Electrode Active Material Mixing Step

Lithium-nickel composite hydroxide and lithium hydroxide monohydrate(available from Wako Pure Chemical Industries, Ltd.) were weighed, andthe resulting composite hydroxide particles and the lithium compoundwere mixed in such a manner that the molar ratio among the metalelements of lithium-nickel composite oxide becameNi:Co:Mn:Li=0.50:0.20:0.30:1.08.

Firing Step

The resulting mixture was fired using an electric furnace in an airatmosphere at 900° C. for 9 hours. Then, the mixture was cooled to roomtemperature in the furnace and then cracked to give a positive electrodeactive material consisting of spherical lithium-nickel-cobalt-manganesecomposite oxide particles which were agglomerations of primaryparticles.

Evaluation of Positive Electrode Active Material

This positive electrode active material was embedded in a resin andsubjected to cross-section polishing, and a cross-section thereof wasobserved using a 5000×SEM. As a result, the positive electrode activematerial was confirmed to have a hollow structure including a shellconsisting of sintered primary particles and a hollow inside the shell.The porosity of the positive electrode active material obtained fromthis observation was 58%. Using this positive electrode active material,a battery was produced in the following manner. The physical propertyresults of the precursor are shown in Table 1, and the physical propertyresults of the active material are shown in Table 2. The composition andimpurity amounts of the resulting positive electrode active material areshown in Table 2.

Method for Producing Battery

With 90 parts by weight of the positive electrode active material powderwere mixed 5 parts by weight of acetylene black and 5 parts by weight ofpolyvinylidene fluoride, and n-methylpyrrolidone was added to make themixture pasty. The pasty positive electrode active material was appliedto a 20-μm-thick aluminum foil in such a manner that the weight of theactive material became 0.05 g/cm2 when dried, dried in a vacuum at 120°C., and then punched into a 1-cm-diameter disc serving as a positiveelectrode.

Lithium metal was used as a negative electrode, and an equivalent mixedsolution of ethylene carbonate (EC) and diethyl carbonate (DEC) using 1Mof LiClO₄ as a supporting electrolyte was used as an electrolytesolution. A separator formed of polyethylene was impregnated with theelectrolyte solution, and a 2032-type coin battery was produced in aglove box containing an Ar gas atmosphere whose dew point was controlledto −80° C. FIG. 1 shows a schematic structure of the 2032-type coinbattery. The coin battery includes a positive electrode (evaluationelectrode) 1 in a positive electrode can 5, a lithium metal negativeelectrode 3 in a negative electrode can 6, a separator 2 impregnatedwith the electrolyte solution, and a gasket 4. Properties (the dischargecapacity, coulomb efficiency, reaction resistance) of the battery areshown in Table 2.

Example 2

A positive electrode active material was prepared as in Example 1 exceptthat composite hydroxide particles were washed with 50 g/L of an aqueoussolution of 0.47 mol/L sodium carbonate having a pH of 11.8 (at areference temperature of 25° C.), and then evaluated. The evaluationsare shown in Tables 1 and 2.

Example 3

A positive electrode active material was prepared as in Example 1 exceptthat composite hydroxide particles were washed with 50 g/L of an aqueoussolution of 1.12 mol/L sodium carbonate having a pH of 12.0 (at areference temperature of 25° C.), and then evaluated. The evaluationsare shown in Tables 1 and 2.

Example 4

A positive electrode active material was prepared as in Example 1 exceptthat the molar ratio among the metal elements in the mixed aqueoussolution was Ni:Co:Mn=60:20:20 in a nuclei formation step for producinga precursor, and then evaluated. The evaluations are shown in Tables 1and 2.

The composite hydroxide particles prepared had a pore structure,consisted of spherical secondary particles having an average particlediameter of 9.8 μm which were agglomerations of primary particles havinga diameter of 1 μm or less, and had a porosity of 46%. For thecomposition thereof, the molar ratio among nickel, cobalt, and manganesewas 60:20:20. The positive electrode active material had a hollowstructure or porous structure and a porosity of 49%.

Example 5

A positive electrode active material was prepared as in Example 1 exceptthat the molar ratio among the metal elements in the mixed aqueoussolution was Ni:Co:Mn=40:10:50 in a nuclei formation step for producinga precursor and that a nitrogen gas was circulated at 5 L/min throughoutthe nuclei formation step and a particle growth step so that the oxygenconcentration of the space in the reaction vessel became 0.2% by volumeor less, and then evaluated. The evaluations are shown in Tables 1 and2.

The composite hydroxide particles prepared had a pore structure,consisted of spherical secondary particles having an average particlediameter of 10.1 μm which were agglomerations of primary particleshaving a diameter of 1 μm or less, and had a porosity of 32%. For thecomposition thereof, the molar ratio among nickel, cobalt, and manganesewas 40:10:50. The positive electrode active material had a hollowstructure or porous structure and a porosity of 27%.

Comparative Example 1

A positive electrode active material was prepared as in Example 1 exceptthat composite hydroxide particles were washed with an aqueous solutionof 0.09 mol/L sodium carbonate having a pH of 11.0 (at a referencetemperature of 25° C.), and then evaluated. The evaluations are shown inTables 1 and 2.

Comparative Example 2

A positive electrode active material was prepared as in Example 1 exceptthat composite hydroxide particles were washed with an aqueous solutionof 1.60 mol/L sodium hydroxide having a pH of 13.5 (at a referencetemperature of 25° C.), and then evaluated. The evaluations are shown inTables 1 and 2. Lithium-nickel composite oxide was prepared, and abattery was produced using the lithium-nickel composite oxide. Theresults are shown in Table 1.

Comparative Example 3

A positive electrode active material was prepared as in Example 1 exceptthat composite hydroxide particles were washed with an aqueous solutionof 3.39 mol/L sodium hydroxide having a pH of 14.0 (at a referencetemperature of 25° C.), and then evaluated. The evaluations are shown inTables 1 and 2.

Comparative Example 4

A positive electrode active material (lithium-nickel-cobalt-manganesecomposite oxide) was prepared as in Example 1 except that:crystallization was effected by using a reaction vessel for continuouscrystallization having an overflow pipe in an upper part thereof,continuously adding a mixed aqueous solution similar to that in Example5, an aqueous ammonia solution, and an aqueous sodium hydroxide solutionat a constant flow rate while keeping the 25° C.-based pH at a constantvalue of 12.5, setting the average residence time in the vessel to 10hours, and continuously collecting the overflowing slurry; after theinside of the reaction vessel reached equilibrium, the slurry wascollected and separated into solid and liquid components; the productwas washed with water, filtered, and dried to give nickel compositehydroxide particles; and no washing step was performed after the firingstep. Then, the positive electrode active material prepared wasevaluated. The evaluations are shown in Tables 1 and 2.

No pore structure was observed in the composite hydroxide particlesprepared; the composite hydroxide particles had a dense particlestructure, consisted of spherical secondary particles having an averageparticle diameter of 8.1 μm which were agglomerations of primaryparticles having a diameter of 1 μm or less, and had a porosity of 3%.For the composition thereof, the molar ratio among nickel, cobalt, andmanganese was 0.40:0.10:0.50. The positive electrode active material hada dense particle structure and a porosity of 2%.

TABLE 1 composition of concentration amount of impurities compositehydroxide porosity washing g/L (mass %) particles (%) solution (mol/L)SO₄ Na Cl Example1 Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂ 51 sodium 30 0.290.028 0.013 carbonate (0.28) Example2 Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂51 sodium 50 0.26 0.025 0.012 carbonate (0.47) Example3Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂ 51 sodium 120 0.21 0.021 0.007carbonate (1.12) Example4 Ni_(0.60)Co_(0.20)Mn_(0.20)(OH)₂ 46 sodium 300.29 0.025 0.01 carbonate (0.28) Example5Ni_(0.40)Co_(0.10)Mn_(0.50)(OH)₂ 32 sodium 30 0.26 0.027 0.014 carbonate(0.28) Comparative Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂ 51 sodium 10 0.460.034 0.025 Example1 carbonate (0.09) ComparativeNi_(0.50)Co_(0.20)Mn_(0.30)(OH)₂ 51 sodium 65 0.58 0.036 0.26 Example2hydroxide (1.6) Comparative Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂ 51 sodium136  0.49 0.041 0.25 Example3 hydroxide (3.39) ComparativeNi_(0.40)Co_(0.10)Mn_(0.50)(OH)₂ 3 sodium 30 0.09 0.015 0.011 Example4carbonate (0.28)

TABLE 2 firing composition of amount of impurities discharge coulombreaction temperature composite oxide porosity (mass %) capacityefficiency resistance (° C.) particles (%) SO₄ Na Cl (mAh/g) (%) (Ω)Example1 900 Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 58 0.27 0.030 0.003159 88.5 3.2 Example2 900 Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 57 0.240.026 0.003 162 88.9 3.4 Example3 900Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 58 0.18 0.023 0.003 163 88.7 3.1Example4 860 Li_(1.08)Ni_(0.60)Co_(0.20)Mn_(0.20)O₂ 49 0.27 0.027 0.003175 89.1 2.9 Example5 930 Li_(1.08)Ni_(0.40)Co_(0.10)Mn_(0.50)O₂ 27 0.250.029 0.004 160 88.4 3.4 Comparative 900Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 58 0.45 0.035 0.006 153 87.6 3.8Example1 Comparative 900 Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 57 0.560.038 0.018 157 86.9 3.6 Example2 Comparative 900Li_(1.08)Ni_(0.50)Co_(0.20)Mn_(0.30)O₂ 58 0.48 0.043 0.006 157 88.2 3.6Example3 Comparative 930 Li_(1.08)Ni_(0.40)Co_(0.10)Mn_(0.50)O₂ 2 0.090.017 0.002 158 87.9 3.8 Example4

Table 1 indicates that the nickel-manganese composite hydroxideparticles and lithium-nickel-manganese composite oxide particles ofExamples 1 to 5 had structures where many pores were present within theparticles (including porous structures/hollow structures) but containedvery small amounts of impurities. Also, the positive electrode activematerials prepared had excellent battery characteristics, such as highcapacity, high coulomb efficiency, and low reaction resistance.

On the other hand, the composite hydroxide particles and composite oxideparticles of Comparative Examples 1 to 3 had structures where many poreswere present within the particles, as well as contained large amounts ofimpurities and had low capacity and high reaction resistance becausethey were not washed sufficiently.

As for the composite hydroxide particles of Comparative Example 4 had adense particle structure and were sufficiently washed with sodiumcarbonate and therefore contained a smaller amount of impurities thanExamples. However, the positive electrode active material thereof had aless pore structure and therefore contacted an electrolyte solution in asmaller area and exhibited lower battery characteristics than Example 5having a similar composition ratio.

INDUSTRIAL APPLICABILITY

The precursor of a positive electrode active material for nonaqueouselectrolyte secondary batteries and the positive electrode activematerial for nonaqueous electrolyte secondary batteries according to thepresent invention contain a significantly reduced amount of impurities,and a nonaqueous electrolyte secondary battery using this positiveelectrode active material has high capacity and excellent coulombefficiency and reaction resistance. For this reason, this battery ispreferred particularly as a rechargeable secondary battery used in thefield of small electronic devices and has extremely high industrialapplicability.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 positive electrode (evaluation electrode)    -   2 separator (impregnated with electrolyte solution)    -   3 lithium metal negative electrode    -   4 gasket    -   5 positive electrode can    -   6 negative electrode can

1. A positive electrode active material for nonaqueous electrolytesecondary batteries, consisting of lithium-nickel-manganese compositeoxide particles represented by the general formula (2), wherein thepositive electrode active material has a sulfate group content of 0.4%by mass or less and a sodium content of 0.035% by mass or less, thelithium-nickel-manganese composite oxide particles have a hollowconsisting of a central space and a shell outside the hollow or poresare distributed throughout the particles, and wherein a porosity of thelithium-nickel-manganese composite oxide particles is from 49 to 58%,Li_(a)Ni_(x)Co_(y)Mn_(z)M_(t)O₂  general formula (2) where 0.95≤a≤1.20;0.2≤x≤0.8; 0≤y<0.3; 0.07<z≤0.8; 0≤t≤0.1; x+y+z+t=1; and M is at leastone element selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo, Hf, Ta,and W.
 2. The positive electrode active material for nonaqueouselectrolyte secondary batteries of claim 1, wherein the positiveelectrode active material has a chlorine content of 0.05% by mass orless.
 3. A nonaqueous electrolyte secondary battery comprising thepositive electrode active material for nonaqueous electrolyte secondarybatteries of claim
 1. 4. The positive electrode active material fornonaqueous electrolyte secondary batteries of claim 1, wherein0.1≤y≤0.25.
 5. The positive electrode active material for nonaqueouselectrolyte secondary batteries of claim 1, wherein 0.35≤x≤0.6.
 6. Thepositive electrode active material for nonaqueous electrolyte secondarybatteries of claim 1, wherein 0.1≤z≤0.5.
 7. The positive electrodeactive material for nonaqueous electrolyte secondary batteries of claim1, which has a porous structure.